Under present conditions, petroleum refineries are finding it necessary to convert increasingly greater proportions of crude to premium fuels including gasoline and middle distillates such as diesel and jet fuel. Catalytic cracking processes, exemplified by the fluid catalytic cracking (FCC) process and Thermofor catalytic cracking (TCC) process together, account for a substantial fraction of heavy liquids conversion in modern refineries. Both are thermally severe processes which result in a rejection of carbon to coke and to residual fractions; during catalytic cracking high molecular weight liquids disproportionate into relatively hydrogen-rich light liquids and aromatic, hydrogen-deficient heavier distillates and residues.
Hydrocracking may be used to upgrade the higher-boiling more refractory products derived from catalytic cracking prior to the treatment in the present hydrotreating process. Useful hydrocracking catalysts include the catalysts taught in U.S. Pat. No. 5,227,353, the most prominent among which is a metallosilicate identified as MCM-41 which is usually synthesized with Brensted acid sites by incorporating a tetrahedrally coordinated trivalent element such as Al, Ga, B, or Fe within the silicate framework.
Catalytic cracking therefore produces significant quantities of highly aromatic, light and middle distillates which not only have high sulfur and nitrogen levels, but which may contain as much as 80 wt. % or more of aromatics. Generally, the level of heteroatom contaminants increases with the boiling point of the fraction. For example, the light cycle oil produced as a typical FCC main column bottoms stream contains about 80% aromatics, 4.6% sulfur compounds, 1500 ppm nitrogen compounds, and about 9.1% hydrogen (in proportions and percentages by weight, as in the remainder of this specification unless otherwise defined).
Present market requirements make highly aromatic product streams such as these particularly difficult to dispose of as commercially valuable products. Formerly, the light and heavy cycle oils could be upgraded and sold as light or heavy fuel oil, such as No. 2 fuel oil or No. 6 fuel oil. Upgrading the light cycle oil was conventionally carried out by a relatively low severity, low pressure catalytic hydrodesulfurization (CHD) unit in which the cycle stock would be admixed with virgin middle distillates from the same crude blend fed to the catalytic cracker.
At many petroleum refineries, the light cycle oil (LCO) from the FCC unit is a significant component of the feed to the catalytic hydrodesulfurization (CHD) unit which produces No. 2 fuel oil or diesel fuel. The remaining component is generally virgin kerosene taken directly from the crude distillation unit. The highly aromatic nature of LCO, particularly when the FCC unit is operated in the maximum gasoline mode, increases operational difficulties for the CHD and can result in a product having marginal properties for No. 2 fuel oil or diesel oil, as measured by cetane numbers and sulfur content. Further, increasingly stringent environmental regulations limiting the aromatics content of diesel fuel have prompted refiners to focus research efforts on economical methods for producing the required low-aromatics fuels.
An alternative market for middle distillate streams is automotive diesel fuel. However, diesel fuel has to meet a minimum cetane number specification of about 45 in order to operate properly in typical automotive diesel engines. Because cetane number correlates closely and inversely with aromatic content, the highly aromatic cycle oils from the cracker typically with aromatic contents of 80% or even higher have cetane numbers as low as 4 or 5. In order to raise the cetane number of these cycle stocks to a satisfactory level by the conventional CHD technology described above, substantial and uneconomic quantities of hydrogen and high pressure processing would be required.
Thus from an economic and operational standpoint, it would be desirable to rely upon the CHD unit for desulfurization, and to provide a more effective and less costly method for reducing aromatics content while providing a product which closely matches the boiling ranges of the feedstock.